JAVA Annotation(注解):java.lang.annotation

使用注解功能时，如果需要用反射读取注解，就必须设置@Retention(RetentionPolicy.RUNTIME)，因为默认情况下为CLASS，读取的时候会报异常

@Retention可选参数：RetentionPolicy

@Target可以限制注解针对的对象：

上面两个注解在我们写注解的时候常用的，如果需要查看注解的详细说明，请看这里：

​​http://www.blogjava.net/mlh123caoer/archive/2007/09/06/143260.html​​

下面是我写的一个简单例子，通过注解，自动生成创建表的SQL

先是两个注解：@SqlTable和@SqlColumn

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface SqlTable {
  public String name();
}

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SqlColumn {
  //列名
  public String name() default "";
  //长度
  public int length() default 0;
  //是否允许空
  public boolean notNull() default false;
  //类型通过反射获取
}

一个用来测试的简单类：TableTest

@SqlTable(name="tableTest")
public class TableTest {
  @SqlColumn(name="username",length=20,notNull=true)
  private String name;
  @SqlColumn(name="age")
  private Integer ageInteger;
  @SqlColumn()
  private Date birthday;
  @SqlColumn(name="bz",length=200,notNull=false)
  private String beizhu;
  
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  public Integer getAgeInteger() {
    return ageInteger;
  }
  public void setAgeInteger(Integer ageInteger) {
    this.ageInteger = ageInteger;
  }
  public Date getBirthday() {
    return birthday;
  }
  public void setBirthday(Date birthday) {
    this.birthday = birthday;
  }
}

处理注解的方法：

public class SqlCreateTable {
  @SuppressWarnings("rawtypes")
  public static String CreateTableSql(Object obj) throws Exception{
    
    String tablename = null;
    //通过isAnnotationPresent判断是否存在注解
    if(obj.getClass().isAnnotationPresent(SqlTable.class)){
      //获取类的注解
      SqlTable sqlTable = obj.getClass().getAnnotation(SqlTable.class);
      //获取注解参数值
      tablename = sqlTable.name();
    }
    else {
      tablename = obj.getClass().getName();
    }
    //获取全部字段
    Field[] fields = obj.getClass().getDeclaredFields();
    
    StringBuffer createTable = new StringBuffer("create table ").append(tablename).append("(\n");
    StringBuffer fieldBuffer = new StringBuffer();
    
    for(Field field:fields){
      //获取注解
      if(field.isAnnotationPresent(SqlColumn.class)){
        
        SqlColumn sqlColumn = field.getAnnotation(SqlColumn.class);
        
        String name = sqlColumn.name();
        if(name.equals("")){
          name = field.getName();
        }
        
        String type = null;
        String Null = "";
        
        boolean notNull = sqlColumn.notNull();
        if(notNull){
          Null = "not null";
        }
        
        Integer length = sqlColumn.length();
        
        Class fieldType = field.getType();
        
        if(fieldType==String.class){
          type = "varchar2";
          if(length==0){
            length = 50;
          }
        }
        else if(fieldType == Integer.class){
          type = "number";
          if(length==0){
            length = 12;
          }
        }
        else if(fieldType == Date.class){
          type = "date";
        }
        else {
          throw new Exception("字段类型错误!");
        }
        //合并字符串
        StringBuffer columnBuffer = new StringBuffer("\t");
        columnBuffer.append(name).append(" ").append(type);
        if(type.equals("date")){
          columnBuffer.append(Null);
        }else {
          columnBuffer.append("(").append(length).append(")").append(" ").append(Null);
        }
        if(!fieldBuffer.toString().equals("")){
          fieldBuffer.append(",\n");
        }
        //加入到字段列表
        fieldBuffer.append(columnBuffer.toString());
      }
    }
    createTable.append(fieldBuffer.toString()).append("\n)");
    return createTable.toString();
  }
  
  
  
  
  /**
   * 测试
   * @param args
   */
  public static void main(String[] args) {
    
    TableTest tableTest = new TableTest();
    try {
      String sql = SqlCreateTable.CreateTableSql(tableTest);
      System.out.println(sql);
    } catch (Exception e) {
      e.printStackTrace();
    }

  }

}

输出结果：

create table tableTest(
  username varchar2(20) not null,
  age number(12) ,
  birthday date,
  bz varchar2(200) 
)